1. Field of Use
The present invention concerns a multiuser detection method and device. More particularly, the present invention concerns a maximum likelihood multiuser detection method and device for an MC-CDMA (Multi-Carrier Code Division Multiple Access) telecommunication system.
2. Discussion of the Background
Multicarrier Code Division Multiple Access (MC-CDMA) combines OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique was proposed for the first time by N. Yee et al. in an article entitle “Multicarrier CDMA in indoor wireless radio arrays” which appeared in Proceedings of PIMRC '93, Vol. 1, pages 109–113, 1993. The developments of this technique were reviewed by S. Hara et al. in the article entitled “Overview of multicarrier CDMA” published in IEEE Communication Magazine, pages 126–133, Dec. 1997.
Unlike the DS-CDMA (Direct Sequence Code Division Multiple Access) method in which the signal of each user is multiplied in the time domain in order to spread its frequency spectrum, the signature here multiplies the signal in the frequency domain, each element of this signature multiplying the signal of a different sub-carrier.
More precisely, FIG. 1 depicts the structure of an MC-CDMA transmitter for a given user k. Let dk(i) be the ith symbol to be transmitted from the user k, where dk(i) belongs to the modulation alphabet. The symbol dk(i) is first of all multiplied at 110 by a spread sequence or signature of the user, denoted ck(t), consisting of Nc “chips”, each “chip” being of duration Tc, the total duration of the spread sequence corresponding to a symbol period T. The results of the multiplication of the symbol dk(i) by the different “chips” are converted by the serial to parallel converter 120 into a block of L symbols, where L is in general a multiple of Nc. It will be considered, for reasons of simplification of presentation, that L=Nc. The block of L symbols is then subject to an inverse FFT in the module 130 in order to be transmitted to the parallel to serial converter 140. In order to prevent inter-symbol interference, a guard time, with a length greater than the duration of the pulse response of the transmission channel, is added to the MC-CDMA symbol. This guard time is obtained by the addition (not shown) of a suffix chosen so as to be identical to the start of the said symbol. The symbol thus obtained is amplified at 150 before being transmitted over the channel of the user. It can therefore be seen that the MC-CDMA method can be analysed as a spread in the spectral domain (before IFFT) followed by an OFDM modulation.
In practice, the user k transmits his data in the form of frames of N symbols, each symbol dk(i) being spread by a real signature ck(t) with a duration equal to the symbol period T, such that ck(t)=0 if t∉[0,T]. The signal modulated at time t=i.T+n.Tc can then be written, if the guard times between the MC-CDMA symbols are omitted:
                                          S            k                    ⁡                      (            t            )                          =                              ∑                          i              =              0                                      N              -              1                                ⁢                                          ⁢                                    ∑                              n                =                0                                            L                -                1                                      ⁢                                                  ⁢                                          a                k                            ·                                                d                  k                                ⁡                                  (                  i                  )                                            ·                                                c                  k                                ⁡                                  (                                      n                    ·                                          T                      c                                                        )                                            ·                              exp                ⁡                                  (                                                            j                      ·                      2                                        ⁢                    π                    ⁢                                                                                  ⁢                                          n                      /                                              N                        c                                                                              )                                                                                        (        1        )            where ak is the amplitude of the signal transmitted by the user k.If the case is now taken of a base station transmitting symbols to K users, the resulting modulated signal can be expressed simply as:
                              S          ⁡                      (            t            )                          =                              ∑                          k              =              1                        K                    ⁢                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          ∑                                  n                  =                  0                                                                      N                    i                                    -                  1                                            ⁢                                                          ⁢                                                a                  k                                ·                                                      d                    k                                    ⁡                                      (                    i                    )                                                  ·                                  c                  kn                                ·                                  exp                  ⁡                                      (                                                                  j                        ·                        2                                            ⁢                      π                      ⁢                                                                                          ⁢                                              n                        /                                                  N                          c                                                                                      )                                                                                                          (        2        )            where ckn=ck(n.Tc) has been noted.
An MC-CDMA receiver for a given user k has been illustrated schematically in FIG. 2.
The demodulated received signal is sampled at the “chip” frequency and the samples belonging to the guard time are eliminated (elimination not shown). The signal obtained can be written:
                              R          ⁡                      (            t            )                          =                                            ∑                              k                =                1                            K                        ⁢                                          ∑                                  i                  =                  0                                                  N                  -                  1                                            ⁢                                                          ⁢                                                ∑                                      n                    =                    0                                                        L                    -                    1                                                  ⁢                                                                  ⁢                                                      a                    k                                    ·                                                            d                      k                                        ⁡                                          (                      i                      )                                                        ·                                      c                    kn                                    ·                                                            h                      kn                                        ⁡                                          (                      i                      )                                                        ·                                      exp                    ⁡                                          (                                                                        j                          ·                          2                                                ⁢                        π                        ⁢                                                                                                  ⁢                                                  n                          /                                                      N                            c                                                                                              )                                                                                                    +                      η            ⁡                          (              t              )                                                          (        3        )            where hkn(i) represents the response of the channel of the user k at the frequency of the sub-carrier n of the MC-CDMA symbol transmitted at time i.T and η(t) is the noise received.
The samples thus obtained are put in parallel by a serial to parallel converter 210 before undergoing an FFT in the module 220. The samples in the frequency domain, output from 220, are equalised and despread by the signature of the user k. To do this, the samples of the frequency domain are multiplied by the coefficients qkn(i).c*kn in the multipliers 23000, . . . , 230Nc−1 and then added at 240 in order to supply an output symbol {circumflex over (d)}k(i).
Different possibilities of equalisation have been envisaged in the state of the art, including notably the MRC (Maximium Ratio Combining) combination defined by the user of the coefficients qkn(i)=h*kn(i) or .* is the conjugation operation.
The receiver illustrated in FIG. 2 makes it possible to decode only the data of a single user k. However, it is often advantageous to decode data transmitted for all of the K users, in order to estimate and eliminate the interference between the different transmission channels. The techniques of multiuser detection and conjoint detection have also been envisaged for the MC-CDMA method. For example, the article by S. Kaiser et al. entitled “Multi-carrier CDMA with iterative decoding and soft-interference cancellation” published in GLOBECOM '97 at pages 6–10, 1997, proposes a method of conjoint detection with parallel elimination of the interference (PIC standing for Parallel Interference Cancellation). However, this detection technique does not necessarily provide the optimum solution in terms of maximum likelihood. In addition, the direct application of a maximum likelihood detection technique to a multiuser context would result in prohibitive complexity.